Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at $4.46\mathrm{\mu m}$ ). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of $3\times {10}^{-9}\text{/cm}$ for $1\mathrm{s}$ averaging of spectra composed by 100 independent points. At $4.46\mathrm{\mu m}$ , this yields a detection limit of 35 parts in ${10}^{12}$ by volume for ${\mathrm{N}}_2\mathrm{O}$ at $50\mathrm{mbar}$ , corresponding to $4\times {10}^7\text{molecules}/{\mathrm{cm}}^3$ , or still to $1\mathrm{fmol}$ in the sample volume.